1. Field of the invention
The present invention relates to a method of preventing ionic penetration into a gate oxide, more specifically, a method of preventing fluorine ions from residing in a gate to result in boron ion penetration into a gate oxide.
2. Description of the Prior Art
A metal-oxide semiconductor (MOS) is a very common electrical device in integrated circuits. A gate, a source, and a drain together comprise the MOS transistor to form a unit with four nodes. By utilizing channel effects generated by the gate of the MOS under different gate voltages, the MOS is often made to function as a digital solid switch. Both increasing complexity and precision in the development of integrated circuits has made controlling the manufacturing process of MOS transistors an important issue.
Please refer to FIG. 1 to FIG. 5 of the cross-sectional views of forming a MOS transistor according to the prior art. As shown in FIG. 1, a semiconductor wafer 10 has an oxide layer 14, a conductive layer 16, and an anti-reflection coating (ARC) 18 composed of silicon-oxy-nitride (SiOxNy) and formed by a plasma-enhanced chemical vapor deposition (PECVD) process positioned, respectively, on a substrate 10.
As shown in FIG. 2, a patterned photoresist layer 20 is formed in a predetermined area on the ARC 18 to define patterns of a gate 22. As shown in FIG. 3, an etching process is performed to remove portions of both the ARC 18 and the conductive layer 16 uncovered by the photoresist layer 20. The gate 22 and the gate oxide layer 24, composed of the remaining portions of the conductive layer 16 and the oxide layer 14 respectively, are thus formed on the surface of the substrate 12.
As shown in FIG. 4, after stripping both the photoresist layer 20 and the ARC 18, a first ion implantation process is performed using the gate 22 as a hard mask and boron fluoride (BF2+) as the dopant to form lightly doped drains (LDD) 26 of the MOS transistor in the substrate 12 adjacent to the gate 22. A spacer 27 is then formed around the gate 22.
Finally, as shown in FIG. 5, a second ion implantation is performed using the gate 22 and the spacer 27 as a hard mask and boron (B) as the dopant to form a source 28 and a drain 29 of the MOS transistor in the substrate 12 adjacent to the spacer 27.
However, when the first ion implantation process is performed using the gate 22 as a hard mask and boron fluoride (BF2+) as the dopant to form the LDD 26 of the MOS transistor in the substrate 12 adjacent to the gate 22, fluorine ions can easily penetrate into the gate 22. In the subsequent second ion implantation process, the boron ions also penetrate into the gate 22. Thus, threshold voltage change, leakage current, and capacitance-voltage curve distortion due to boron ion penetration into the gate oxide layer 24, are enhanced by the fluorine ions that have previously penetrated into the gate 22 so as to cause a defect in the MOS transistor.
It is therefore a primary object of the present invention to provide a method of preventing fluorine ions from residing in a gate to result in boron ion penetration into a gate oxide.
In the preferred embodiment of the present invention, a substrate, an oxide layer, a conductive layer, an anti-reflection coating (ARC), and a photoresist layer defining patterns of a gate are formed, respectively, on a semiconductor wafer. An etching process is performed to remove portions of both the ARC and the conductive layer uncovered by the photoresist layer to form the gate and a gate oxide layer composed of the residual conductive layer and the oxide layer, respectively. After the photoresist layer is stripped, an ion implantation process is performed using the gate covered by the ARC as hard mask and boron fluoride (BF2+) as the dopant to form lightly doped drains (LDD) in the substrate adjacent the gate. After the ARC is removed, a spacer is formed around the gate. Finally, the method is completed with the formation of a source and a drain in the substrate adjacent to the spacer after the ARC is stripped.
It is an advantage of the present invention against the prior art that the LDD of the MOS transistor is formed by performing the first ion implantation process, using instead the gate covered by the ARC as a hard mask and boron fluoride (BF2+) as the dopant. Therefore, fluorine ion penetration, leading to boron ion penetration into the gate oxide and causing threshold voltage change, leakage current, and capacitance-voltage curve distortion, are prevented.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the multiple figures and drawings.